Plastic molded articles are widely used as bodies of various articles, particularly, electric goods and electronic goods. The plastic molded article, in general, is used in the state which a bending load is added to the plastic molded article. Accordingly, it is necessary to measure the displacement, distortion, and stress when the bending load is added. In this case, since the stress cannot be directly measured, in general, it is measured based on the conversion obtained from the relationship with the distortion.
In general, the basic equation for analyzing the mechanical characteristics of the structure of the molded article is defined by the strength of the materials. According to the strength of the materials, the mechanical characteristic is obtained by the balance of the force, the relationship between the displacement and the distortion, and the relationship between the stress and the distortion in the very small area dA.
The displacement, distortion, stress, and force are important factors in the evaluation of the mechanical characteristics. In this case, the displacement and the distortion are expressed as a geometrical quantity, and the force and the stress are expressed as a dynamic quantity.
In general, the specifications of the plastic molded article are defined by the maximum displacement obtained at the moment when an external force having, for example, an intensity of 1 N (one Newton) is applied to the molded article. That is, the maximum displacement is defined by a predetermined value (for example, below 1 mm) and, the mechanical characteristics are mainly defined by the displacement. Further, the intensity of the molded article is evaluated by the amount of distortion or stress applied by an external force to the molded article. As is known, when stress (distortion) over a predetermined value is applied to the molded article, the molded article is destroyed. The amount of stress (distortion) that can be applied to the molded article varies depending on the material and the structure of the molded article.
Conventionally, there are two methods for evaluating the mechanical characteristic of a plastic molded article. One is a method using the special test piece, and the other is a method using the plastic molded article. The first method is performed by using a small test piece having a predetermined size which is cut off from the molded article. The height, width, and thickness of the test piece is defined by a national standard. The second method is performed based on the predetermined specification as explained above regarding the molded article.
For example, as explained above, the displacement is measured by adding the predetermined pressing force to the test point on the plastic molded article to be tested. Usually, two or three test points are defined on the molded article to shorten the measuring time. Accordingly, in the conventional method, it is very troublesome and difficult to perform measurement by adding the pressing force to various optional points besides the test point, and to quickly measure the displacement at that point.
Further, in the measurement of distortion, in general, a strain gauge is adhered to the molded article to be measured, and a change in the electrical characteristics of the strain gauge is measured before and after adding the pressing force. However, the range of this measurement is limited to only a special measuring point to which the strain gauge is attached. Further, it is troublesome to perform the adhesion and the wiring of the strain gauge so that additional time is required to measure the distortion.
In general, a plastic molded article has characteristics which change in accordance with molding conditions, such as molding temperature, and molding pressure. Accordingly, to obtain the desired characteristics of the molded article, it is necessary to evaluate of the characteristics at optional points based on the conditions of the actual plastic mold. In this case, it is necessary to quickly perform measurement of displacement, distortion, and stress to actually evaluate the deformation state and the added pressing force.
To satisfy the above requirements, the present inventor proposes a robot measuring system which can measures automatically and precisely measure the mechanical characteristics of a plastic molded article in the deformation state when adding the pressing force.